This invention relates to printers such as line printers, and more particularly to a device for limiting the number of printing columns (hereinafter referred to as "a printing column number limiting device" when applicable) in such a printer.
Printing in a line printer is carried out as follows: A group of printing data codes transferred from a data source are successively compared with a group of type data codes which are determined in correspondence to the instantaneous positions of types on a type carrier. When a printing data code coincides with a type data code position, a printing hammer in the place where the coincidence is obtained is driven for printing of that type.
A group of printing data codes are transferred in various modes from the data source. In a particular mode, a phenomenon may exist where the printing data codes coincide with the type codes for all the printing places within a limited short period of time. This is a "worst case" condition in terms of structual requirements on a system. In this case, the printing hammers for all the printing places are driven collectively within the short period of time. Accordingly, the printer should be provided with a rigid mechanical construction to withstand this collective driving of the printing hammers and have a corresponding large capacity printing hammer driving electric source.
However, in normal operation of the printer, it is rare that a group of printing data codes will be transferred in the aforementioned particular mode from the data source. Therefore, if the mechanical construction and electric source are designed by taking this rare condition into account, then the printer will become considerably more expensive than necessary for ordinary use.
In order to overcome this difficulty in the prior art a technique is provided for a printer in which a group of printing data codes are compared with a group of type data codes successively for every type pitch movement and thus printing operations for printing places are performed. In a situation where the printing data codes coincide with type data codes to be carried out at substantially the same time, a printing column number limiting circuit is employed in which a means is established for monitoring the number of printing columns and printing is inhibited after the number of printing columns having the coincidence of the printing data codes and the type codes has reached a predetermined number.
The means for monitoring the number of printing columns where such coincidences are obtained is a counter to count coincidence signals concerning the printing data codes and the type codes. The counter is cleared with every type pitch movement. This printing column number limiting circuit is one prior art technique of solving the above-described problem. It is still disadvantageous in one aspect:
In the case where the printing hammer driving period (hereinafter referred to as "a hammer pulse width" when applicable) is longer than one type pitch movement time of the type carrier, the overload of the hammer electric source cannot be positively prevented without reducing the predetermined printing speed.
This is illustrated in FIG. 1. It is assumed that when the count value of the counter counting the number of printing columns where printing data codes coincide with type codes for every type pitch movement reaches a value "N", printing operations for printing columns where such coincidences are obtained thereafter are not carried out. In this case, printing hammers for "N" places become operable whenever a hammer pulse width is equal to one type pitch movement time tc as shown in FIG. 1. If the hammer pulse width is represented by .tau., tc &lt; .tau. .ltoreq. 2tc, the peak value of current flowing in a printing hammer is "I" amperes. The peak value of current supplied by the hammer electric source (hereinafter referred to as "a hammer electric source current" when applicable) may reach (I.times.N.times.2) amperes.
In other words, even if the number of columns for the printing hammers which can be driven during one type pitch movement is limited to the value "N", a load of twice the normal is applied to the hammer electric source in the worst case, and therefore, the hammer electric source should have a capacity for withstanding this maximum load.
As indicated, it is rare in normal operation of a printer that the printing hammers for "N" columns are successively driven for every pitch movement. If, under this worst case condition, the number of columns for which the printing hammers can be driven for every type pitch movement is limited to N/2 so that the peak value of the hammer electric source current be smaller than (I.times.N) amperes, then the frequency of causing this condition will markedly increased. As a result the printing speed of the system will be considerably decreased.
As the printing speed of the printer is increased, the one type pitch movement time becomes much shorter than the hammer pulse width .tau., while the peak value of the hammer electric source current is increased. Accordingly, an increase of the capacity of the hammer driving electric source is required. Thus, this method cannot be satisfactorily applied to a high-speed printer.